A tree chipper, also known as a wood chipper, is a special piece of equipment used to change wood and brush into wood chips. Most tree chippers are portable and mounted on wheels so they can be towed behind a truck. In the case of paper making, however, the wood chipper is much larger and is generally a semi-permanent feature at the paper mill.
Most tree chippers contain a hopper with a collar, a chipper mechanism, and a bin for collecting the resulting chips. To start the process, a tree limb or trunk is inserted into the hopper. The collar serves to protect the person inserting the wood from getting cut by the chipping blades. After the wood is passed through the blades and chipped, the chips leave through a chute and go into a container for easy transport to the digester. Typically, the chips are one to two inches in size.
The first tree chippers, which are still made today, were drum-based. The chipping mechanism in one of these tree chippers is in a large steel drum and powered by a motor. Most of the time, a belt supplies this power. The steel drum is located parallel to the hopper and it spins toward the chute. At the same time, the drum draws new materials in to be chipped. This type of tree chipper is sometimes referred to as a chuck-and-duck chipper because materials placed inside begin moving almost immediately after they come in contact with the drum.
The drum-style chipper is not as commonly found in the paper making industry as it once was because it is not as safe as newer designs. The chances of the operator becoming snagged by the materials being placed inside and consequently being fed into the machine are too great. In addition, drum-style chippers are very loud and produces inconsistently sized wood chips. Materials can also become easily stuck in the drum.
The disk tree chipper is a newer design that utilizes a steel disk containing mounted knives. The wheels inside the chipper draw the material to the hopper and toward the disk. These wheels are hydraulically powered and reversible in order to move materials that have become damaged back out.
The disk inside the tree chipper is mounted at a perpendicular angle in relation to the incoming material. As the material approaches, the disk spins and the knives cut the material. The resulting chips are thrown out the chute by flanges located on the drum. Although this design is not as energy efficient as the drum-style tree chipper, it produces more uniformly sized wood chips.
The wood chippers generally used by pulp and paper mills are larger machines called whole tree chippers, or recyclers. These tree chippers may use drums, disks, or both. They are capable of chipping wood two to six feet in diameter. Even bigger tree chippers, called grinders, are capable of chipping wood more than eight feet in diameter by using special hardened hammers to tear the wood up instead of cut it.
Despite the variance in the type of tree chippers, the knives used in them are all basically the same. The knives are generally anywhere from four to six inches across and range from six to twelve inches in length. They have a rectangular shape and can vary from one-half to two inches in thickness. A high-grade steel is used to make chipper knives.
Tree chippers are an important component in the paper making process. Without them, the wood would not be broken down into smaller, uniform pieces and, therefore, would not be able to be added to the digester where they are softened and turned into pulp to be made into paper.